The proposed research will examine the possibility that female mice are masculinized by the presence of other fetuses in utero. Preliminary data have revealed that when all fetuses but one are removed from the uterus in Day 8 of pregnancy, and if that fetus is female, it will look and act more "female-like' than a female having resided contiguous to no male fetuses in a uterus containing its normal complement of fetuses. Masculinization thus may occur in the absence of contiguity to males. The propose of the research will extend the initial finding by examining the display of sexual activity, attractivity, and avoidance responding in the single female fetus. It also will determine whether masculinization depends upon the presence of male fetuses, and if so, how many as well as when during fetal development masculinization occurs. An attempt also will be made to assess levels of testosterone in the blood and amniotic fluid of the single-female. A final experiment will assess the influence of the uterine environment upon the guinea pig, a species normally giving birth to litters of few offspring.